[sgen] Fix check-nursery-pinned debug option (mono/mono#15865)

[sgen] Fix check-nursery-pinned debug option (mono/mono#15865)

* [sgen] Remove dead unpin_queue argument

* [sgen] Fix check-nursery-pinned debug option

And rename it to better reflect what is is doing now (which is to check that we don't leave any vtables tagged at the end of a collection). It used to also check that nursery objects are pinned, back when we were doing a nursery collection as part of the finishing pause of a concurrent collection.

Commit migrated from https://github.com/mono/mono/commit/87ba12e13f98a24eb140e933876a25b45fd806b8